1. Field of the Invention
This invention relates to a high resolution position sensitive radiation detector system, as for example may find extensive but not exclusive utilization in gel electrophoresis.
2. Description of the Prior Art
Experiments in biology often involve the analysis of distributions of radioactively labeled molecules. Such distributions are obtained, for example, in gel electrophoresis when a mixture of electrically charged molecules (proteins, RNA, DNA) migrates through a thin layer of viscous medium (gel) under the influence of an electric field. According to their mobilities, different types of molecules migrate to different positions in the gel forming distinct bands. Two aspects of the information contained in an "electrophoretogram" are of interest: (a) the location of the bands which identifies the molecules and (b) the number of radioactive molecules in each band.
Electrophoretograms are generally analyzed for this information either by slicing the original gel and dissolving the individual pieces in a flour for scintillation counting, or by exposing the gel to a photographic emulsion (autoradiography). Gel slicing yields quantitative results but the resolution in the position of the bands is limited by the size of the slice which is usually 1 mm. Autoradiography offers excellent position resolution (typically .+-.0.2 mm, limited by the thickness of the sample and of the film) but does not yield a direct quantitative measurement of the relative intensity of different bands. Both methods of analysis require several hours to several days.
An apparatus for autoradiography with a position sensitive counter has been reported previously by Gabriel et al., FEBS Letters 39, 307 (1974). Position localization in that apparatus was accomplished by determining the difference in rise time of the signals arriving at the two ends of a resistive anode wire. The spatial resolution achieved was 3 mm. In the past few years, methods for determining the avalanche position along the anode wire of a proportional counter have received considerable attention. Also, the technique of avalanche localization through the induced pulse on a delay line, on which the present invention is based, has been developed for use in high energy physics experiments, as reported by Breskin et al., Nuclear Instr. and Methods 119, 1 (1974), and Okuno et al. IEEE Trans. Nucl. Sci., NS-24, 213 (1977).
Additional disclosures relating to position-sensitive radiation detectors can be found in the patent literature as represented by U.S. Pat. No. 3,483,377 to Borkowski et al., U.S. Pat. No. 3,517,194 to Borkowski et al., and U.S. Pat. No. 4,149,109 to Kreutz et al. The two Borkowski et al. patents disclose a position-senstitive detector formed of an ionizing radiation detecting element having a very high resistance collector which is of sufficient resistance per unit length to provide a voltage pulse at an output end thereof, wherein the rise time of the voltage pulse is proportional to the distance between the location of an ionizing event and the end of the collector. A low resistance reference electrode is placed in proximity to the high resistance collector electrode such that an ionizing event additionally produces a constant rise time reference pulse independent of impact position in the reference electrode. Then, by time comparison of the position independent reference pulse with the position dependent pulse produced in the high resistance collector electrode, Borkowski et al. determine radiation impact position.
In the above-noted Kreutz et al. patent, a location-sensitive proportional counter tube is provided with a trough-shaped cathode in a counting chamber in which is longitudinally extended a resiliently elastic anode wire. Fasteners engage the end of the anode wire to mount the anode wire equidistantly from the side walls of the cathode trough and are connected in electrical connection with the input resistor of a preamplifier. The counting chamber is pressurized by means of inlet and outlet apertures with a pressurized counter tube gas, for example, 90% argon, 10% methane.
In addition to the above references, further methods and apparata for radiation localization are disclosed in U.S. Pat. Nos. 3,772,521; 3,898,465; 3,975,638; 3,975,639; 4,075,492; and 4,150,290.